Polycarbonates are well known, commercially available materials which have achieved wide acceptance in the plastics industry. Such polymers are prepared by reacting a carbonate precursor such as phosgene, for example, with a dihydric phenol such as 2,2-bis(4-hydroxyphenyl) propane, hereinafter referred to as "bisphenol-A," to provide a linear polymer consisting of dihydric phenol derived units bonded to one another through carbonate linkages. Generally speaking, such polymers offer a high resistance to mineral acids, having a high tensile strength and a dimensional stability and impact strength far surpassing that of any other thermoplastic material.
These polycarbonates are an exception to most thermoplastic polymers in their melt rheology behavior. Most thermoplastic polymers exhibit non-Newtonian flow characteristics over essentially all melt processing conditions. Newtonian flow is defined as the type of flow occurring in a liquid system where the rate of shear is directly proportional to the shearing force. Linear bisphenol-A polycarbonate exhibits Newtonian flow at normal processing temperatures and shear rates below 300 reciprocal seconds.
Two other characteristics of molten thermoplastic polymers are significant for molding and these are melt elasticity and melt strength. Melt elasticity is the recovery of the elastic energy stored within the melt because of distortion or orientation of the molecules by shearing stresses. Melt strength may be simply described as the tenacity of a molten strand and is really the ability of the melt to support a stress.
Both of these are important in extrusion blow molding particularly to fabrication by extrusion blow molding. Non-Newtonian flow characteristics tend to impart melt elasticity and melt strength to such polymers allowing the use thereof in blow molding fabrication. In the usual blow molding operation, a hollow tube of molten thermoplastic is extruded vertically downward at a temperature of about 200.degree.- 400.degree. C. A mold then surrounds the tube and air introduced into the tube forces it to conform to the shape of the mold. The length of the tube and the quantity of material comprising the tube are limiting factors in determining the ultimate size and wall thickness of the molded part. The fluidity of the melt obtained from bisphenol-A polycarbonate, or the lack of melt strength and also the paucity of extrudate swelling, serve to limit blow molding applications to relatively small, thin walled parts. Temperatures must also be extremely carefully controlled to prevent the desired length of extruded tube from falling away before the mold can close around it for blowing. Consequently, it will be appreciated that the Newtonian behavior of polycarbonate resin melts has served to restrict severely their use in the production of large hollow bodies by conventional extrusion blow-molding operations as well as in the production of various shapes by profile extrusion methods.
Thermoplastic randomly branched polycarbonates possess definite and unique properties which allow their use in preparing articles not heretofore easily or readily produced with linear reaction products of a dihydric phenol and a carbonate precursor. The thermoplastic randomly branched polycarbonates are prepared by reacting a polyfunctional compound containing more than 2 functional groups with a dihydric phenol and a carbonate precursor.
Several prior art disclosures such as exemplified by U.S. Pat. Nos. 2,950,266 and 3,030,335 concerning the addition of a trifunctional additive to polycarbonate forming reactions between dihydric phenols and carbonyl halides teach that if a cross-linked product does not occur as a direct result of the initial polymerization reaction, the final reaction product of the dihydric phenol, the trifunctional compound, and the carbonyl halide would be a heat curable product.
Other prior art attempts have been made to incorporate a trifunctional compound into polycarbonates of dihydric phenols as exemplified by U.S. Pat. No. 3,094,508. This has been limited to the preparation of extremely high molecular weight (i.e., in excess of 200,000) polymers having melting points exceeding 300.degree. C., under very limited process conditions such as a temperature range of 28.degree.-34.degree. C, which conditions are deemed necessary to avoid gelation, i.e., formation of an insoluble, infusible product. Aside from the fact that such extremely high molecular weight, high melting polymers have little commercial importance as molding resins, their preparation requires carefully controlled process conditions which are both cumbersome and expensive. In addition, other attempts have been made to provide polycarbonate resins which exhibit non-Newtonian melt characteristics as, for example, disclosed in U.S. Pat. No. 3,166,606. However, the polycarbonates there disclosed consist of a physical blend of two or more polycarbonate resins having differing values of reduced viscosity. Moreover, the individual polycarbonate components disclosed by the patentees in the production of such polycarbonate blends are produced entirely from difunctional reactants.